The invention described herein relates to radiant energy imaging systems, and in particular pulse echo imaging systems having a three-dimensional display capability.
Acoustic imaging techniques are well-known and extensively described in the prior art to provide visual inspection or analysis of all three phase states of materials, i.e., solids, liquids and gases. Examples of application of acoustic imaging techniques include industrial non-destructive evaluation of metals and liquids, medical ultrasound imaging, underwater imaging and echo ranging in the atmosphere. In the development of such techniques several classes of acoustic imaging systems have been devised. These various classes of systems obtain images in one-, two- or three-dimensions. The various types of imaging devices include direct imaging systems such as acoustic cameras or pulse-echo devices and indirect reconstructed imaging devices including acoustic holographic devices, synthetic aperture devices and computed tomographic systems.
Fundamentally, acoustic imaging devices perform two basic tasks: the acquisition of and the display of that data in a human readable form. Whatever of the various techniques used, a provision must be made for the latter two functions.
In order to provide a one-dimensional image the most prevalent technique is the pulse-echo A-mode technique in which a piezoelectric transducer transmits a short burst of acoustic energy into a medium, and then receives and displays the amplitudes of the echoes as a function of the echo range, i.e., the time of flight.
Of the known two-dimensional imaging techniques, the most significant is the pulse-echo B-mode tomography technique in which echoes returning to the transducer are displayed as brightness levels proportional to echo amplitude. The transducer generally used for such purposes is capable of being mechanically or electronically translated or steered in one dimension. In the display the brightness levels are displayed with reference to echo range and transducer position or orientation providing cross-sectional images of the object.
In order for there to be a better understanding of the known prior art imaging techniques reference should be had to FIG. 1 of the drawings. In this figure is shown a one-dimensional transducer array E,F which can produce a B-mode scan indicated as a portion of the object volume, E, F, W, V. B-mode images are comprised of many B-mode lines obtained at the rate of one B-mode line per transmitted acoustic pulse. The maximum B-mode line rate is given by: EQU R.sub.(lines/sec) =V/2Z
where V is the acoustic propogation velocity and Z is the maximum range of the image. Recent developments in this field have included parallel signal processing techniques which enable one to obtain and display several B-mode image lines per acoustic pulse.
Orthographic projection imaging systems form a third significant class of acoustic imaging techniques. Devices of this nature include C-mode pulse-echo scanners, acoustic three-dimensional scanning systems, transmission or reflection acoustic cameras and acoustic holographic imaging systems. In these devices a three-dimensional volume of the object is interrogated by acoustic radiation using either "floodlight" insonification or beam formed pulses. Data from the volume can then be processed and displayed in several different ways. For C-mode imaging systems a single transducer or transducers operates in the pulse-echo mode. The transducers are mechanically or electronically scanned using a rectangular raster format so that a three-dimensional volume of the object is interrogated by the ultrasonic beam. Alternatively, the front surface of the transducer is fixed at a single point and the body of the transducer is moved in a spiral motion so that the transducer insonifies a conical three-dimensional volume in a spiral format.
In each case only echo data from a preselected range is displayed as brightness levels proportional to echo amplitude. Due to the use of a fixed focus lens and an electronic range gate, a C-scan device presents two-dimensional data in an orthographic display in which the display coordinates are the x,y cartesian coordinates of the targets at a fixed depth in the object.
To illustrate the above, in FIG. 1 a single element of the two-dimensional transducer array A,B,C,D is fired and receives echoes from one line of the three-dimensional volume. Only those echoes which are located in a predetermined range gate are displayed in a single image point. After each element of the array has been fired subsequently, the complete C-mode image will be obtained, for example, in the plane RSTU. Due to the fact that the display does not include target range, but includes directions perpendicular to target range, C-mode systems operate so that each point in the image requires a transmitted acoustic pulse. Thus, the time necessary to develop a complete C-mode image is significantly longer than the image formation time for B-mode image, and for example, an N.times.M C-scan requires N.times.M transmitted ultrasound pulses.
While the above described systems are not particularly useful for producing three-dimensional images, the prior art includes pulse-echo scanned three-dimensional imaging systems. One such system causes a transducer to be scanned in a raster format insonifying a three-dimensional rectangular parallelopiped. Cartesian coordinates are used in the display in a complicated manner which allows an orthographic display of a three-dimensional object in different projections but without image perspective. Again referring to FIG. 1, the three-dimensional volume using this system is interrogated as in the case of the C-scan, but in this case the echoes from the entire volume are displayed as a function of the X,Y coordinates so that parallel object planes in the Z direction overlap in the image.
Another prior art system utilizes a three-dimensional scanner in which a conical volume is insonified by a combination of sector steering plus rotation. Again, in this system cartesian coordinates are used in the display, although it is claimed that some perspective is obtained by modulating the size of the X,Y display with the third cartesian coordinate Z.
In each of the types of so-called three-dimensional scanning systems a line of pulse-echo data along one transducer orientation is displayed as a single point in the image. Accordingly, the time required to form the complete three-dimensional image is identical to the time required to develop a C-mode image. This amount of time is significantly greater than that required for the formation of a B-mode image. Therefore, while these systems produce a greater amount of information the time required for the production of that information is similarly greatly increased. Moreover, proper three-dimensional perspective is probably not achieved.
Transmission and reflection acoustic cameras these have been developed for medical and underwater imaging applications. The latter devices include a fixed focus acoustic lens and a receiving array of transducers in a water bath which limits their applications. The acoustic cameras are somewhat analogous to optical cameras. The receiving transducers function as a sampled film plane for an image formed by an acoustic lens. In these devices a range gate and the depth of focus of the lens restrict data to a fixed predetermined range resulting in an orthographic projection image similar to an acoustic C-scanner.
Additional prior art systems which offer prospects of producing three-dimensional images include acoustic holographic systems. These have the capability of obtaining data from a three-dimensional object by interference of an object wave and a reference pattern, i.e., a hologram. The three-dimensional image must then be optically reconstructed or reconstructed through the use of a computer from the hologram. This is a relatively complex and expense way of achieving this kind of data.
It is therefore an object of this invention to provide an improved radiant energy imaging system capable of producing an image of a three-dimensional object having an improved three-dimensional perspective relative to presently available techniques.
Another object of this invention is to provide an improved radiant energy pulse-echo imaging system capable of producing an image of a three-dimensional object with improved three-dimensional perspective.
A further object of this invention is to provide an improved radiant engery pulse-echo imaging system capable of producing an image of a three-dimensional object having improved three-dimensional perspective utilizing a two-dimensional dispaly having such perspective.
Still another object of this invention is to provide an improved acoustic imaging system capable of producing an image of a three-dimensional object in a two-dimensional display having perspective and having the capability in the two-dimensional transducer array of steering the transmit and receive orientations to predetermined orientations.
An additional object of this invention is to provide a system such as those described hereinabove having additionally the capability of range discrimination wherein such range discrimination can be accomplished using a range dependent gain control, brightness shading as a function of range or a color display having differing hues corresponding to differing ranges.